A microwave wave generator device with a virtual cathode oscillator conventionally comprises a diode constituted by a cathode and an anode, emitting a beam of electrons, as well as a cylindrical wave guide. The anode is generally constituted by a thick frame and by a thin sheet (frequently called “thin anode” below by simplification). By “thin” it is meant here that the sheet of the anode has a thickness of a few tenths of micrometers. As regards the thin sheet, it is coupled to the cylindrical wave guide. In other words, the thin anode separates the cathode from the wave guide, at the interface between the thick frame and the wave guide, and, furthermore, the thick armature generally surrounds the cathode.
This type of device is known to produce high power pulses of microwaves
To that end, a potential difference is applied to the terminals of the diode creating an electronic emission at the location of the cathode. At the location of the thin sheet of the anode, the components of the electric field that are transverse relative to a longitudinal axis of the wave guide cancel out, the electron beam begins to be pinched under the effect of its magnetic field. When the current entering the cylindrical wave guide exceeds the space-charge current limit, the electron density becomes so great that the beam can no longer propagate within the wave guide. An accumulation of charge, commonly called “virtual cathode”, then forms behind the thin sheet. The virtual cathode then deviates numerous electrons to the extent of sending some back towards the cathode, through the thin sheet. While approaching the thin anode, the virtual cathode increases its charge density until the time at which it disintegrates under the effect of its own space-charge and a new virtual cathode reconstitutes a little further in the wave guide. This is the oscillation principle of the virtual cathode which is at the origin of a microwave wave emission.
Such a device is compact and of simple design. Its operation is robust and does not require recourse to an external magnetic field. However its power efficiency (ratio of maximum power of the emitted wave to the maximum electrical power input into the diode) is very low, of the order of 1%. Furthermore, the frequencies of the emitted wave directly follow the temporal variations in the applied voltage, which leads to an electromagnetic wave being obtained of mediocre spectral quality.
To counter at least some of these drawbacks while maintaining an axial geometry, the implantation of one or more reflectors in the cylindrical wave guide has been proposed. This type of device was the subject of patent application WO2006037918.
The reflectors are thin walls (that is to say a few tenths of micrometers thick), transparent to electrons and configured to reflect the microwave wave created by the virtual cathodes. Furthermore, generally they are of circular cylindrical shape.
This type of device with reflectors enables substantially improved performance to be obtained relative to the devices without reflector. However, there is an optimum number of reflectors beyond which the power efficiency decreases.